WO2009123227A1 - Steel material, process for producing steel material, and apparatus for producing steel material - Google Patents

Abstract

A process for producing a steel material is provided which includes subjecting a highly strengthened steel material (W) to a heat treatment to thereby cause part of the steel material to have a lower hardness than the remainder of the steel material (W).  The heat treatment comprises: a heating step in which that part of the steel material (W) which ranges from the surface to a certain depth is rapidly heated by induction heating or direct ohmic heating; and a cooling step in which the steel material (W) which has undergone the heating step is rapidly cooled at a given time period after the heating step.  In the heating step, the steel material (W) is heated to a temperature not lower than transformation point Ac1.

Description

Steel material, steel material manufacturing method and steel material manufacturing apparatus

The present invention relates to a steel material, a steel material manufacturing method, and a steel material manufacturing apparatus, and more particularly to a material whose hardness varies depending on a part.

For example, in order to improve delayed fracture resistance in secondary processing (heat treatment) from a steel material such as a coiled rolled material (hereinafter referred to as a rolled material), which is a raw material of a bar or wire, for example, a region from the surface layer to the center A technique for changing the tensile strength (hardness) is provided.

For example, a technique is known that uses ultra-low carbon steel such as pure iron on the surface layer and rolls it by decarburization and decarburization, such as clad steel (see, for example, JP-A-4-212570) ).

Also, a technique using a decarburized layer formed on the surface layer by heating at a temperature of Ac1 to Ac3 during rolling (see, for example, Japanese Patent Application Laid-Open No. Sho 62-267420), and after the heat treatment, heat treatment is performed again only on the surface layer portion. Techniques (see, for example, JP-A-7-54441) are provided.

Furthermore, in general, as a means for improving delayed fracture resistance, there is a technique for performing chemical surface treatment such as plating or nitriding, or a technique for applying a non-metallic coating material with excellent delayed fracture resistance to the surface. Are known.

However, the above techniques have the following problems. That is, pre-treatment is necessary because secondary processing (heat treatment) is performed after producing a decarburized layer by producing clad steel or heating at a temperature of Ac1 to Ac3 at the stage of the rolled material. In addition, after secondary processing (heat treatment), it is necessary to heat-treat only the surface layer portion or perform chemical surface treatment again. Therefore, the process is complicated in all cases, and complicated processing conditions must be controlled before and after the secondary processing (heat treatment).

Therefore, an object of the present invention is to provide a steel material, a steel material manufacturing method, and a steel material manufacturing apparatus that have different hardness depending on the site by a simple process.

A method for manufacturing a steel material according to one aspect of the present invention is a method for manufacturing a steel material in which the hardness of a part of the steel material is made lower than the hardness of the other part of the steel material by performing a heat treatment on the steel material having increased strength. The heat treatment includes a heating step of rapidly heating the steel material from a skin to a certain depth by induction heating or direct current heating, and a rapid cooling of the steel material after the heating step after a predetermined time after the heating step. A cooling step, wherein the heating temperature in the heating step is equal to or higher than the Ac1 transformation point.

As another aspect of the present invention, the time from the heating step to the cooling step is not more than a predetermined time determined according to the steel type, wire diameter, heating temperature, and heating time.

As another aspect of the present invention, the heat treatment conditions are determined based on heat transfer characteristics after rapid heating of the surface of the steel material.

In another aspect of the present invention, the treatment condition of the heat treatment is a time integral value of the temperature of the steel material, and is determined based on a tempering progress value indicating a progress state of the tempering of the steel material. And

In another aspect, the present invention is characterized in that the heat treatment conditions include a combination including at least two of frequency, input electric energy, heating temperature, heating time, and cooling time.

The present invention includes, as another aspect, a step of calculating the heat transfer characteristic of the steel material or the tempering progress value, and the heat treatment condition is set to the calculated heat transfer characteristic or the tempering progress value. It is determined based on.

In another aspect of the present invention, the heat treatment conditions are set so that the tempering progress value in the surface layer portion is 1.5 times or more the tempering progress value in the center portion. Features.

In another aspect of the present invention, the time from the heating step to the cooling step is such that the tempering progress value in the surface layer portion is 1.5 times or more the central tempering progress value. It is characterized by setting as follows.

As another aspect of the present invention, the steel material is linear or rod-shaped.

In another aspect of the present invention, after the steel material is subjected to a quenching process including a heat treatment and a cooling process, the heating process and the cooling process are each performed once as a tempering process. Features.

Another embodiment of the present invention is the steel material that has undergone the heating step and the cooling step, and the difference between the hardness in the vicinity of the surface layer portion and the hardness on the center side from the position of 10% from the surface layer in the radial direction. Is HV50 or more, and the tensile strength when a tensile test is performed with a No. 2 test piece of JISZ2201 is 1420 N / mm ² or more.

As another aspect of the present invention, the entire cross-section tempered martensite structure is obtained, the hardness of the surface layer portion is HV380 or less, and a tensile test is performed with a No. 2 test piece of JISZ2201. The tensile strength is 1420 N / mm ² or more, and the hardness on the center side of the surface layer is uniform.

As another aspect of the present invention, when the entire cross-section tempered martensite structure is formed, the hardness of the surface layer portion is HV420 or less, and a tensile test is performed with a No. 2 test piece of JISZ2201 The tensile strength is 1600 N / mm ² or more, and the center side hardness is more uniform than the surface layer.

Another embodiment of the present invention is an apparatus for manufacturing a steel material, wherein the hardness of a part of the steel material is made lower than the hardness of the other part of the steel material by performing a heat treatment on the strengthened steel material, In the heating step, a heating means that rapidly heats the steel material from the skin to a certain depth by induction heating or direct current heating, and a cooling means that rapidly cools the heated steel material after a predetermined time after the heating, The steel manufacturing apparatus is characterized in that the heating temperature is equal to or higher than the Ac1 transformation point.

As another aspect of the present invention, the present invention is further characterized by further comprising control means for controlling processing conditions of the heat treatment based on a heat transfer characteristic of the steel material or a calculation result of the tempering progress value.

FIG. 1 is an explanatory view schematically showing the configuration of a heat treatment apparatus according to the first embodiment of the present invention. FIG. 2 is an explanatory view schematically showing a heat treatment step of the same embodiment. FIG. 3 is a side view showing the configuration of the PC steel rod in the same embodiment. FIG. 4 is a table showing components of the PC steel bar in the same embodiment. FIG. 5 is a graph showing the relationship between the number of coil turns and the wire diameter in the heat treatment apparatus of the same embodiment. FIG. 6 is a table showing the heat treatment conditions of the embodiment and the conventional example. FIG. 7 is a graph showing the relationship between the elapsed time and the temperature distribution by heat transfer analysis of the PC steel rod of the same embodiment. FIG. 8 is a graph showing a relationship between a cooling time and a temperature distribution by heat transfer analysis of the PC steel rod of the same embodiment. FIG. 9 is a graph showing the relationship between the hardness of the PC steel rod of the same embodiment and the distance in the diameter direction. FIG. 10 is a graph showing the temperature distribution from the surface layer portion to the center in the tempering process in the same embodiment. FIG. 11 is a graph showing the efficiency of the N parameter in the same embodiment. FIG. 12 is a graph showing the relationship between the ratio of the N parameter between the surface and the center, the heating process and the cooling time in the same embodiment. FIG. 13: is a graph which shows the cross-sectional hardness distribution of the PC steel bar obtained by the heat processing which concerns on the same embodiment, and the conventional heat processing. FIG. 14 is a graph showing the cross-sectional hardness distribution in the axial direction of the PC steel bar manufactured by the heat treatment of the same embodiment. FIG. 15 is a table showing the components of a plurality of types of PC steel bars used for the heat treatment of the same embodiment. FIG. 16 is a table showing the delayed fracture test results of the PC steel bar W by the heat treatment of the same embodiment and the conventional heat treatment. FIG. 17 is a graph showing the delayed fracture test results. FIG. 18 is a side view showing the configuration of the PC steel bar W1 of the same embodiment. FIG. 19 is a side view showing the configuration of the notch of the PC steel bar W1 of the same embodiment. FIG. 20 is a table showing the delayed fracture test results of the PC steel bar by the heat treatment of the same embodiment and the conventional heat treatment. FIG. 21 is a graph showing the delayed fracture test results. FIG. 22 is a graph showing the relationship between the depth of the notch formed in the PC steel bar W1 of the same embodiment and the fracture time. FIG. 23 is an explanatory view schematically showing a configuration of a heat treatment apparatus according to another embodiment of the present invention. FIG. 24 is a side view showing the configuration of the deformed PC steel bar according to the second embodiment of the present invention. FIG. 25 is a table showing components of the deformed PC steel bar in the same embodiment. FIG. 26 is a table showing heat treatment conditions for heat treatment and comparative heat treatment according to the embodiment. FIG. 27 is a graph showing a simulation result by heat transfer analysis of elapsed time and temperature change in heat treatment of the deformed PC steel bar in the same embodiment. FIG. 28 is a perspective view showing a configuration of a spring steel wire according to a third embodiment of the present invention. FIG. 29 is a table showing components of the spring steel wire in the same embodiment. FIG. 30 is a table showing heat treatment conditions in the heat treatment and the comparative heat treatment according to the embodiment. FIG. 31 is a graph showing a simulation result by heat transfer analysis of elapsed time and temperature change in the heat treatment according to the embodiment. FIG. 32 is a graph showing the cross-sectional hardness distribution of the spring steel wire after the heat treatment according to the embodiment and the comparative heat treatment. FIG. 33 is a graph showing the relationship between the hardness of the heat-treated material and the fatigue limit. FIG. 34 is a table showing the results of the rotational bending fatigue test of the comparative heat-treated material. FIG. 35 is a graph showing the relationship between the distance from the surface layer of the comparative heat-treated material and the number of durability. FIG. 36 is a table showing the rotational bending fatigue test results of the spring steel wire by the heat treatment according to the embodiment. FIG. 37 is a graph showing the cross-sectional hardness, residual stress, and stress amplitude of the spring steel wire by the heat treatment and the spring steel wire by the comparative heat treatment material according to the embodiment. FIG. 38 is a table showing an example of components of a spring steel wire according to another embodiment. FIG. 39 is a perspective view showing a configuration of a bolt according to the fourth embodiment of the present invention. FIG. 40 is a table showing the components of the bolt according to the embodiment. FIG. 41 is a table showing the heat treatment conditions in the heat treatment and the comparative heat treatment according to the embodiment. FIG. 42 is a graph showing a simulation result by heat transfer analysis of elapsed time and temperature change in the heat treatment of the bolt Wb in the same embodiment. FIG. 43 is a graph showing the cross-sectional hardness distribution of the bolts obtained by the heat treatment according to the embodiment and the comparative heat treatment. FIG. 44 is a table showing the delayed fracture test results of the bolts obtained by the heat treatment according to the embodiment and the comparative heat treatment. FIG. 45 is a graph showing the cumulative failure probability and failure time of the bolts obtained by the heat treatment according to the embodiment and the comparative heat treatment.

Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. 1 to 22. FIG. 1 is a conceptual diagram of a heat treatment apparatus 10 of the present embodiment. FIG. 2 is a flowchart of the manufacturing process of the PC steel bar in the present embodiment.

As shown in FIG. 1, a heat treatment apparatus 10 as an example of a steel material manufacturing apparatus includes a pinch roll 11 (conveying means) that conveys a PC steel bar W that is an example of a steel material, and a quenching heating coil that is a quenching means. 12, a quenching cooling jacket 13, a pinch roll 14 (conveying means), a tempering heating coil 15 as a heating means for performing high-frequency induction heating, a cooling jacket 16 as a cooling means, and a pinch roll 17 (conveying means). Are arranged along a linear conveyance path, and have a function of heating and cooling while conveying a PC steel rod (steel material) W along the conveyance path.

The PC steel bar W to be processed is, for example, a solid round bar as shown in FIG. 3 and is continuously conveyed along the axial direction. The PC steel bar W is configured to include the components shown in FIG. 4, for example, but is not limited thereto.

The tempering heating coil 15 has a function of high-frequency induction heating of the PC steel rod W passing there between. The tempering heating coil 15 is set to an appropriate number of coil turns in accordance with the wire diameter and the conveyance speed. The number of turns of the tempering heating coil 15 in this embodiment is, for example, 6 turns, but is not limited to this. As a comparison object, a conventional product using a 17-turn coil is illustrated. FIG. 5 shows the relationship among the general number of coil turns, wire diameter, and conveyance speed.

In the tempering by high frequency induction heating, the PC steel rod W itself generates heat. The depth of the heat generating portion depends on the number and frequency of coil turns of the tempering heating coil 15, input electric energy, heating temperature, heating time, and cooling time. It can be adjusted by the combination.

The cooling jacket 16 has a function of injecting a cooling liquid onto the passing PC steel rod W to cool it.

The distance from the tempering heating coil 15 to the cooling jacket 16 is set to 500 mm or less, for example. In a normal processing apparatus, this distance is about 1900 mm. However, in this embodiment, the distance between the heating process and the cooling process is set short in order to shorten the time.

An example of the heat treatment conditions in this embodiment is shown in FIG. This processing condition was discovered by the applicant based on the principle described later, using the temporal change of the heating temperature pattern at the moment of high-frequency induction heating and the tempering characteristics of steel. By continuously heat-treating under these treatment conditions, it has a low hardness layer in the surface layer part by one tempering and has a uniform hardness distribution from a certain depth, and a strength of 1420 N / mm ² or more It becomes possible to manufacture the steel rod W which becomes a level.

The processing conditions of this embodiment are a frequency of 50 kHz, a quenching heating temperature of 1000 ° C., a tempering heating temperature of 805 ° C., a tempering heating time of 0.17 s, and a time from tempering heating to cooling of 0.63 s. The PC steel bar used here is a thin PC steel bar having a diameter d (nominal name) of 7.1 mm, and the tempering heating temperature is adjusted so that the tensile strength is about 1440 N / mm ² .

The processing conditions of the conventional product to be compared are a frequency of 9.5 kHz, a quenching heating temperature of 1000 ° C., a tempering heating temperature of 603 ° C., a tempering heating time of 0.59 s, and a time from tempering heating to cooling of 3.48 s. This conventional product is also a small-diameter PC steel rod having a diameter d (nominal name) of 7.1 mm, and the tempering heating temperature is adjusted so that the total tensile average tensile strength is about 1440 N / mm ² . In addition, let the component of this embodiment and a conventional product be the same component.

That is, the tempering heating temperature of this embodiment is higher than that of the conventional product to be compared, and the time from tempering heating to cooling is set shorter than that of the conventional product.

As for the tempering temperature, according to the conventional common sense, heating at the tempering temperature not lower than the Ac1 transformation point (727 ° C.) is “impossible because tempering occurs”. By using heating and controlling rapid heating of the surface and rapid cooling immediately after completion of heating, it is possible to prevent quenching by combining rapid cooling even at temperatures above the Ac1 transformation point. I found out.

In the heat treatment apparatus 10 according to the present embodiment, based on the principle described later, the feed speed by the pinch roll 11 or the like, the heating temperature by the tempering heating coil 15, the heating time, the distance between the tempering heating coil 15 and the cooling jacket 16 , Etc. can be appropriately set and adjusted to obtain a PC steel bar W having a desired hardness distribution.

Hereinafter, the operation of the heat treatment apparatus 10 configured as described above will be described with reference to the flowchart of FIG. A continuous wire W0, which is a continuous wire or rod-shaped steel material drawn in the material process, is continuously conveyed from the left side to the right side in FIG. 1 by a pinch roll 11 (conveying means). The continuous wire W0 to be conveyed is rapidly heated to the quenching temperature by induction heating by the quenching heating coil 12 in the quenching process, and then rapidly quenched by quenching cooling liquid injection by the quenching cooling jacket 13 and continuously quenched. Is done.

The continuous wire W0 subjected to the quenching process is heated while passing through the tempering heating coil 15. The continuous wire W0 heated to a predetermined tempering temperature is conveyed to the cooling jacket 16 to which the cooling liquid is continuously sprayed, and the cooling jacket 16 starts rapid cooling. Passing through the cooling jacket 16, the entire length of the continuous wire W0 is cooled, and the tempering heat treatment is completed. And W0 is carried out by the pinch roll 17. After completing the heat treatment, the continuous wire W0 (PC steel bar W) is processed into a product (PC steel bar W) through a processing / inspection process.

Next, the principle for determining the processing conditions in the heat treatment will be described.

Hereinafter, the N parameter value (tempering progress value), which is a new parameter serving as a reference for setting the processing conditions, will be described.

Fig. 7 shows the simulation results by heat transfer analysis using a finite element model (FEM) of the elapsed time and temperature change from the start of heating of a cross section of a steel material during continuous heat treatment to 0.8 s. At six points from the surface to the center, the elapsed time is shown on the horizontal axis and the temperature is shown on the vertical axis. Here, the heating time is 0.17 s, so out of 0.8 s in the graph, the first (left side of graph) 0.17 s is the heating time, and the remaining 0.63 s on the right side of the graph is allowed to cool. It will be time.

As a general tempering parameter, the Larson Miller parameter P = T × (A + logt) [T: temperature (K), A: constant, t: time (h)] established in the case of long-time heating is known. ing. As the value of the parameter P increases, the tempering progresses (low hardness).

On the other hand, when the high-frequency induction heating is short-time heating, for example, when a rapid heating process and a rapid cooling process in the high-frequency heat treatment as shown in FIG. 7 are established, a certain N value can be set as a parameter. The N value (N parameter value) to be defined is the time integral of the temperature of the PC steel bar W, and is represented by the following formula (1).

Here, T: temperature (° C.), t: time (s), and t0: heat treatment time (s).

That is, this N parameter value is an area enclosed below each curve in the graph. As the value of the N parameter value increases, the tempering progresses (low hardness).

According to FIG. 7, when t0 is 0.8, that is, at the time of 0.8 s from the start of heating and when the cooling time is 0.63 s, when the N parameter value indicated by the area is compared, The portion is larger than the N parameter value in the central portion, and the difference in the N parameter hardly changes at a depth of about 2 mm or more. Therefore, if it cools at this time, as shown in FIG. 9, the surface layer part is low-hardness and the state which has a substantially uniform hardness distribution from a low-hardness layer end part to a center part is realizable. In the example of this embodiment, t0 is set to 0.8, but the present invention is not limited to this, and an appropriate value can be applied according to various conditions depending on the wire diameter, steel type, and the like.

FIG. 8 is a graph showing the relationship between the cooling time and the temperature distribution by heat transfer analysis of a PC steel rod using FEM. The horizontal axis of r (distance from the center) / R (radius) and the vertical axis of temperature are shown every 0.3 s. The analysis conditions were: shape: solid round bar, radius: 3.65 mm, material: S40C, heating layer depth 0.154 mm, heating time 0.17 s, cooling time 0.63 s, and initial temperature 20 ° C. In addition, t in a graph is shown by the time from the heating start corresponding to FIG. 7, and has shown the analysis result from 0.17 s which starts cooling to 0.8 s which finishes cooling. Therefore, t = 0.8 s corresponds to the cooling time 0.63 s.

As shown in FIG. 8, the heat generated by the heat generated in the surface layer is transmitted to the center of the PC steel bar and to the outside with time. In the initial stage of heating, the surface layer has a high temperature distribution and the center has a low temperature distribution, but the temperature of the surface layer and the center becomes uniform over time. At the time of t = 0.8 s, the surface temperature Ts = 430 ° C. and the center temperature = 429.1.

FIG. 9 shows the measurement of the cross-sectional hardness of the PC steel bar W, with the horizontal axis indicating the position in the diameter direction and the vertical axis indicating the Vickers hardness. The hardness corresponds to the tensile strength, and the surface layer portion has a low hardness (low strength), and a state having a substantially uniform hardness distribution from the end portion to the center portion of the low hardness layer is realized.

Moreover, according to FIG. 8, since it is in a soaking | uniform-heating state after 0.63 s, the difference between N parameter values of a surface layer part and a center part will disappear, so that time until cooling becomes long. I understand.

FIG. 10 shows the N parameter value and Vickers hardness after 0.8 s from the start of heating of the PC steel bar W. It can be seen that the N parameter value and the Vickers hardness have a symmetric relationship and are in good agreement, and the hardness can be organized by the N parameter.

To summarize the above principle, the tensile strength of the PC steel bar W is determined by the N parameter value, which is the progress of tempering from the surface to the center, so there is a certain range to satisfy the standard. I understand. Further, it is understood that a steel material having desired characteristics can be obtained by controlling the temperature distribution and the time until cooling so that the difference between the N parameter values of the surface layer portion and the central portion is as large as possible within this range. .

That is, by utilizing the temporal change of the heating temperature pattern at the moment of high-frequency induction heating and the tempering characteristics of steel, there is a low-hardness layer on the surface layer portion by tempering once in continuous heat treatment. From the depth, a method for producing a steel material having a uniform hardness distribution and a tensile strength, for example, a strength of 1420 N / mm ² or more can be realized.

Generally, it is known that the lower the tensile strength, the better the delayed fracture resistance. That is, the PC steel bar W having a low hardness portion on the surface is a PC steel bar having both excellent delayed fracture resistance and a predetermined tensile strength, and such a PC steel bar W is obtained by the above method. be able to.

That is, in induction tempering, the depth of the heat generating part can be adjusted by selecting an appropriate coil and frequency, input electric energy, heating temperature, heating time, and cooling time, and a pattern calculated by simulation can be realized. Therefore, by adjusting the time until cooling in high frequency heating, only the surface layer portion can be made low in hardness while satisfying the standard tensile strength of the PC steel bar. In addition, radiation-type external heating, such as furnace heating other than high-frequency heating, does not raise temperature or soak in a short period of time, and continues to heat slowly, so the strength difference in the radial direction due to the tempering characteristics of steel. However, in the case of high-frequency heating, there is an advantage that the internal hardness other than the surface layer portion is made uniform because of a short time such as 1 s or less.

Based on this N parameter, the hardness distribution of the surface layer can be adjusted by one tempering using the temperature distribution (difference in N parameter) peculiar to induction heat treatment, and the standard of tensile strength of steel bars A continuous high-frequency heat treatment line that satisfies the values can be realized.

For example, desired good surface layer softening can be achieved by setting the N parameter of the surface layer to be 1.5 times or more the central N parameter.

Note that the N parameter can be suitably used for steel materials of 100 kg / mm grade ² or higher, and C: 0.1 mass% to 0.5 mass% of ordinary steel is preferable because of tempering temperature limitations. The diameter range for the N parameter to act as a principle is preferably, for example, a range of 5 mm to 40 mm.

That is, the N parameter uses the tempering of high-strength steel, and uses the overshoot due to rapid heating and the rapid transition to soaking due to the heat transfer of the steel, which are characteristic of high-frequency induction heating. When the wire diameter is larger than that, tempering is performed in such a range that the desired surface layer softening can be achieved (the N parameter of the center and the surface layer is 1.5 times or more), and the entire surface is uniform and within the specified strength. It is difficult to do. However, the present invention is not limited to this range, and can be applied to large wire diameters exceeding this range in terms of induction tempering.

Furthermore, this suggests time constraints. That is, as shown in FIG. 11 showing the efficiency of the N parameter, it is preferable to cool in a continuous heating line in a short time considering the state in which the whole is tempered, for example, 0.8 s or less in FIG. I understand that. That is, since the overshoot time is very short, when the N parameter value of the surface layer portion is not 1.5 or more of the central portion, the tempering proceeds with the parameters of the Larson Miller, as is well known. In continuous heat treatment, if it takes 10 s or more, high strength cannot be ensured, and thus the time limitation of the N parameter occurs.

FIG. 11 shows the relationship between the N parameter and Vickers hardness. Since the N parameter value and hardness are determined by the formula in the figure, select an appropriate coil and frequency, input electrical energy, heating temperature, heating time, and cooling time, and estimate the N parameter value by simulation. The desired hardness can be obtained.

Fig. 12 shows the ratio of the N parameter between the center and the surface layer of the PC steel bar W and the elapsed time from the start of heating. From the start of heating to about 1 s, the parameter value was 1.5 times or more, and only the surface layer portion had low hardness. The PC steel bar W obtained by the above heat treatment method has a full-section tempered martensite structure.

FIG. 13 shows the cross-sectional hardness distribution of the PC steel bar W obtained by the above heat treatment method. In FIG. 13, the horizontal axis indicates the distance from the surface layer, and the vertical axis indicates the Vickers hardness. As shown in FIG. 13, the cross-sectional hardness distribution of a normal PC steel bar is uniform, whereas the PC steel bar according to the present invention has a uniform hardness at the center, although the hardness near the surface layer is low. It was confirmed that there was.

FIG. 14 shows the cross-sectional hardness distribution in the axial direction of the PC steel bar manufactured by the heat treatment of this embodiment. About six places where the distance from a surface layer differs, a Vickers hardness is shown on a vertical axis | shaft, respectively, and the distance in an axial direction from a reference point is shown on a horizontal axis. As shown in FIG. 14, it was confirmed that the hardness distribution of the PC steel bar W was substantially constant with respect to the axial direction.

Next, in the case where two types of steels having different components shown in A and B of FIG. 15 are applied as the PC steel bar W, the delay when the heat treatment is performed by the heat treatment method under the conditions shown in FIG. The destructive test results are shown in FIG. In addition, among the steel types A, B, C, and D shown in FIG. 15, the steel types A, B, and C were round PC steel rods, and the steel type D was a deformed PC steel rod. FIG. 17 is a graphical representation of the test results shown in FIG. 16, with the vertical axis representing the fracture time and the horizontal axis representing the cumulative fracture probability. The longer the fracture time, the better the delayed fracture resistance. It shows that.

The delayed fracture test was carried out by applying a load of 1420 × 0.7 N / mm ² while immersed in a 20% NH ₄ SCN solution kept at 50 ° C.

As shown in FIG. 17, it was confirmed that various steel bars subjected to the heat treatment according to the present embodiment were superior in delayed fracture resistance compared to steel bars subjected to a normal heat treatment.

Also, FIG. 17 shows that the delayed fracture resistance is better when the plot is generally on the top (that is, the fracture time is longer). That is, in FIG. 17, it can be seen that the PC steel bar treated under the conditions of the present embodiment is more excellent in delayed fracture resistance than the conventional product.

Furthermore, a sample provided with a notch 20 was prepared for a steel type that does not break even in the delayed fracture test, and a delayed fracture test was performed. The structure of the PC steel bar W1 provided with the notch 20 is shown in FIGS.

Next, in the case where two kinds of steel types having different components shown in C and D of FIG. 15 are applied as the PC steel bar W1, heat treatment is performed by a heat treatment method under the same conditions as the PC steel bar W shown in FIG. FIG. 20 shows the result of the delayed fracture test in the case of applying the above. Further, this cumulative fracture probability is shown in FIG.

The delayed fracture test was performed by applying a load of 1420 × 0.8 N / mm ² in a state of being immersed in a 20% NH ₄ SCN solution kept at 50 ° C. FIG. 21 shows the results shown in FIG. 20 with the fracture time on the vertical axis and the cumulative fracture probability on the horizontal axis. The longer the fracture time, the better the delayed fracture resistance.

FIG. 22 shows the relationship between the depth of the notch 20 formed in W1 and the breaking time. It was confirmed that even the PC steel bar W1 provided with the notch 20 was superior in delayed fracture resistance to the steel bar by this heat treatment compared to the steel bar by normal heat treatment. The vertical axis represents the average rupture time, and the horizontal axis represents the notch 20 depth. It can be seen that the delayed fracture resistance improving effect according to the present embodiment is drastically reduced from the depth of 0.4 mm. Since the diameter of the PC steel rod W1 used this time is 7.2 mm, the effect of this embodiment can be confirmed up to 10% of the sample radius from the surface layer, that is, about 0.36 mm from the surface layer.

According to the PC steel bar, the PC steel bar heat treatment method and the heat treatment apparatus according to the present embodiment, the following effects can be obtained.
By combining surface heating by high-frequency induction heating and tempering performance of steel, a PC steel rod excellent in delayed fracture resistance can be obtained by simple processing. That is, by using the temporal change of the temperature pattern at the moment of heating and the tempering characteristics, it is possible to obtain a steel material having different hardness depending on the part by a simple treatment by a single tempering that satisfies a predetermined processing condition. it can.

The present inventors have found that high-frequency induction heating can apply a temperature of 720 ° C. or higher, which is not generally applied due to quenching, and can form a sufficiently soft surface layer. Furthermore, quenching and tempering heat treatment that rapidly heats at high frequency provides high strength and high toughness compared to heat treatment by ordinary furnace heating. By finding the heat treatment conditions based on the thermal characteristics, it is possible to easily find appropriate conditions corresponding to all steel materials.

Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying constituent elements without departing from the scope of the invention in the implementation stage. For example, specific processing conditions can be appropriately changed according to the steel type of the target steel material, the required strength standard, hardness distribution, apparatus specifications, and the like. Also, the heat treatment conditions to be set are not limited to the above.

Further, like the heat treatment apparatus 101 shown in FIG. 23, in addition to the heat treatment apparatus 10 of the first embodiment, the detection means 21 for detecting various kinds of information about the steel material to be treated and the finite element method described above are used. , 8 for calculating a simulation result, a control unit 23 such as a CPU for controlling each condition of the apparatus 101 according to the simulation result, and various settings according to the control of the control unit 23. The adjusting means 24 for adjusting may be provided so that the heat treatment conditions can be controlled and adjusted based on the information of the corresponding steel material. In this case, for example, the time from tempering to cooling can be controlled by adjusting the speed of the feed mechanism of the pinch rolls 11, 14, and 17 and the position of the cooling jacket 16. Moreover, you may comprise so that other heat processing conditions, such as heating temperature and a frequency, can also be controlled by the control means 23. FIG. In this case, the same effect as that of the first embodiment can be obtained. In addition, you may make it a user input about the information of steel materials.

[Second Embodiment]
Hereinafter, a second embodiment of the present invention will be described with reference to FIGS. In addition, since it is the same as that of the said 1st Embodiment except the point which changed the steel material used as the object of a process into the unusual shape PC steel bar Wc, common description is abbreviate | omitted.

The heat treatment according to the present embodiment is a surface softening treatment, the steel material by this surface softening treatment (here, a deformed PC steel bar Wc) is a surface softening material, the heat treatment as a comparative example to be compared is a comparative heat treatment, and this comparative heat treatment The deformed PC steel bar is referred to as a comparative heat treatment material.

In this embodiment, as shown in FIG. 24, a deformed PC steel rod Wc having a uniform spiral groove continuous in the surface layer was treated as a steel material. As in the first embodiment, the manufacturing apparatus and the manufacturing process were performed by the manufacturing process shown in FIG. 2 using the heat treatment apparatus 10 shown in FIG. That is, since the present embodiment is different only in the shape, component, diameter, etc. of the steel material to be processed, the heat treatment conditions are determined based on the same principle and the same parameters as in the first embodiment.

In the present embodiment, the case where a deformed PC steel bar Wc including the components shown in FIG. 25 is used as a processing target is exemplified, but the present invention is not limited to this.

FIG. 26 shows the heat treatment conditions for the deformed PC steel bar Wc according to the present embodiment and the heat treatment conditions for the comparative heat treatment to be compared. The heat treatment conditions of the deformed PC steel bar Wc of this embodiment (surface softening material) are: frequency 50 kHz, quenching heating temperature 1000 ° C., tempering heating temperature 805 ° C., tempering heating time 0.17 s, tempering heating to cooling The time is 0.63 s. The deformed PC steel bar Wc used here is a deformed PC steel bar having a diameter db (nominal name) of 7.1 mm, and the total cross-sectional tensile strength is adjusted to be about 1400 N / mm ² .

The comparative processing conditions are a frequency of 9.5 kHz, a quenching heating temperature of 1000 ° C., a tempering heating temperature of 603 ° C., a tempering heating time of 0.59 s, and a time from tempering heating to cooling of 3.48 s. This comparative heat-treated material is also a deformed PC steel bar having a diameter of 7.1 mm, and is adjusted so that the average tensile strength of the entire cross section is about 1400 N / mm ² . The component of the deformed PC steel bar Wc of the surface softening material of this embodiment and the component of the comparative heat treatment material are the same components.

FIG. 27 shows a simulation result of the relationship between the elapsed time of heat treatment of the deformed PC steel bar Wc and the temperature change in this embodiment. Here, the relationship between the elapsed time and temperature according to the distance from the surface is shown.

As can be seen from FIG. 27, it can be seen that the area of the hatched portion in the figure is larger in the surface layer than in the center. That is, in this surface layer part, it will be in the state hold | maintained for a long time at high temperature, and hardness will fall large.

Also in this embodiment, the same effect as the first embodiment can be obtained. That is, by combining the surface heating by high frequency induction heating and the tempering performance of steel, a deformed PC steel bar having excellent delayed fracture resistance can be obtained by a simple treatment. In other words, by using temporal changes in the temperature pattern at the moment of heating and tempering characteristics, a deformed PC steel bar with different hardness depending on the site can be obtained by simple tempering that satisfies a predetermined processing condition. Obtainable. Furthermore, the quenching and tempering heat treatment, which is rapidly heated by high frequency for a short time, provides high strength and high toughness as compared with heat treatment by ordinary furnace heating.

[Third Embodiment]
Hereinafter, a third embodiment of the present invention will be described with reference to FIGS. In addition, since it is the same as that of the said 1st Embodiment except the point which used the spring steel wire Ws as an example of the steel materials used as the object of a process, common description is abbreviate | omitted.

The heat treatment according to the present embodiment is a surface softening treatment, the steel material by this surface softening treatment (here, the spring steel wire Ws) is a surface softening material, the heat treatment as a comparative example is a comparative heat treatment, and the spring steel wire by this comparative heat treatment is compared. It is called a heat treatment material.

In this embodiment, the steel material to be processed is the spring steel wire Ws shown in FIG. The heat treatment apparatus and the manufacturing process are the same as those in the first embodiment, and the heat treatment apparatus 10 shown in FIG. 1 is used and the process is performed in the manufacturing process shown in FIG. That is, this embodiment is different only in the components, diameters, and the like of the steel material to be processed. Therefore, the heat treatment conditions are determined using the same principle and the same parameters as in the first embodiment.

In the present embodiment, the case where the spring steel wire Ws including the components shown in FIG. 29 is used as an object to be processed is exemplified, but the present invention is not limited thereto.

FIG. 30 shows heat treatment conditions for the heat treatment according to the present embodiment and a comparative heat treatment as a comparative example. The heat treatment conditions for the surface softening treatment according to the present embodiment are: frequency 50 kHz, quenching heating temperature 950 ° C., tempering heating temperature 789 ° C., tempering heating time 0.4 s, time from tempering heating to cooling 2.6 s. To do.

The spring steel wire Ws used here is a spring steel wire having a diameter ds = 12.0 mm, and the total cross-sectional tensile strength is adjusted to be about 1900 N / mm ² .

Comparative heat treatment conditions as a comparative example are a frequency of 9.5 kHz, a quenching heating temperature of 950 ° C., a tempering heating temperature of 495 ° C., a tempering heating time of 1.7 s, and a time from tempering heating to cooling of 11.1 s.

The comparative heat-treated material is also a spring steel wire having a diameter of 12.0 mm, and the total cross-sectional average tensile strength is adjusted to be about 1900 N / mm ² . The spring steel wire Ws and the comparative heat treatment material of the present embodiment have the same components.

FIG. 31 shows a simulation result of the relationship between the elapsed time of the heat treatment of the spring steel wire Ws and the temperature change in the present embodiment. Here, the relationship between the elapsed time and temperature according to the distance from the surface is shown.

As can be seen from FIG. 31, the area of the hatched portion in the figure is larger in the surface layer than in the center. That is, in this surface layer part, it will be in the state hold | maintained for a long time at high temperature, and hardness will fall large.

FIG. 32 shows the distance from the surface layer and the hardness distribution. The vertical axis represents hardness [HV0.3], and the horizontal axis represents distance [mm] from the surface layer. FIG. 32 shows the hardness distributions of a spring steel wire (comparative heat treatment material) subjected to heat treatment as a comparative example and a spring steel wire Ws (surface softening material) treated under the heat treatment conditions of this embodiment.

The hardness of the comparative heat treatment material hardly changed even when the distance from the surface layer was changed. On the other hand, in the spring steel wire Ws subjected to the heat treatment according to the present embodiment, it can be seen that the hardness changes in the vicinity of the surface layer as the distance from the surface layer increases. That is, Hv <500 in the range of 1 mm from the surface layer.

As shown in FIG. 33, generally, when the hardness is [Hv]> 500, an early breakage starting from inclusions is likely to occur when a fatigue test is performed. Therefore, fatigue characteristics are improved in the spring steel wire Ws (surface softening material) in which the surface layer is softened by processing under the heat treatment conditions of the present embodiment.

FIG. 34 shows the relationship between the distance from the surface layer to inclusions and the number of durability as a result of the rotational bending fatigue test of the comparative heat-treated material as a comparative example. FIG. 35 is a graph showing the relationship between the distance from the surface layer to inclusions and the number of durability times in the comparative heat-treated material. From FIG. 34 and FIG. 35, in the comparative heat treatment material, the tendency that the number of durability increases as the distance from the surface layer to the inclusion increases is recognized.

FIG. 36 shows the relationship between the distance from the surface layer to inclusions and the number of durability in the spring steel wire Ws as a result of the rotational bending fatigue test of the spring steel wire Ws (surface softening material) processed according to the present embodiment.

The results of the rotating bending fatigue test are all exemplified when the rotating bending fatigue test is performed after shot peening. The test conditions were stress amplitude: 700 MPa, rotation speed: 2000 rpm. The test was conducted up to 10 million times. In the graph, “> 1000” indicates that no breakage occurred after 10 million times.

Comparing FIG. 34 and FIG. 36, the spring steel wire Ws (surface softening material) of the present embodiment has a significantly increased durability compared to the spring steel wire (comparative heat treatment material) of the comparative example, and fatigue characteristics. It can be seen that is improved. In the spring steel wire Ws surface softening material of this embodiment, it is recognized that all breakage occurs starting from the surface layer.

FIG. 37 is a graph showing the distribution of cross-sectional hardness, residual stress, and stress amplitude. The horizontal axis shows the distance [mm] from the surface layer. The vertical axis represents hardness [HV0.3], stress amplitude [MPa], and residual stress [MPa].

The distribution of the residual stress is the same in all cases, and after showing the maximum compressive stress in the vicinity of 0.1 mm from the surface layer, it shows the tensile stress at 0.2 mm or more.

As can be seen from this graph, the comparative heat-treated material has high hardness and low toughness in the case of the same stress amplitude, so that the residual stress is in the range of 0.2 to 1.0 mm from the tensile surface layer. Premature breakage occurs.

On the other hand, in the spring steel wire Ws of the present embodiment, which is a surface layer softening material, since the surface layer has low hardness and high toughness, inclusions originate from inclusions in the range close to the surface layer of about 0.2 to 1.0 mm. Generation of fatigue cracks is suppressed. For this reason, the number of times of durability is greatly improved.

Also in this embodiment, the same effect as the first embodiment can be obtained. That is, by combining surface heating by high-frequency induction heating and tempering performance of steel, a spring steel wire having excellent delayed fracture resistance can be obtained by simple processing. That is, by using temporal changes in the temperature pattern at the moment of heating and tempering characteristics, a spring steel wire having different hardness depending on the part is obtained by simple tempering with a single tempering condition that satisfies a predetermined processing condition. be able to. Furthermore, the quenching and tempering heat treatment, which is rapidly heated by high frequency for a short time, provides high strength and high toughness as compared with heat treatment by ordinary furnace heating.

Furthermore, according to the spring steel wire Ws manufactured in the present embodiment, premature breakage starting from inclusions hardly occurs by softening in the range of 0.2 to 1.0 mm from the surface layer where the residual stress is tensile. The effect of doing is obtained.

In addition, in this embodiment, although the component shown in FIG. 29 was shown as an example, it is not restricted to this. Other examples include steel types E to I including components as shown in FIG. Even in the case of these steel materials, the surface layer vicinity can be softened by the same treatment as described above, high strength and high toughness can be obtained, and early breakage starting from inclusions can be prevented. . In addition, the lowest stage of the table | surface of FIG. 38 shows each component contained in the illustrated some spring steel wire in the range.

[Fourth Embodiment]
A fourth embodiment of the present invention will be described below with reference to FIGS. 39 to 45. In addition, since it is the same as that of the said 1st Embodiment except the point which used the volt | bolt Wb as an example of the steel materials used as the object of a process, common description is abbreviate | omitted.

The heat treatment according to this embodiment is referred to as a surface softening treatment, the bolt Wb resulting from this surface softening treatment is referred to as a surface softening material, the heat treatment as a comparative example is referred to as a comparative heat treatment, and the bolt resulting from this comparative heat treatment is referred to as a comparative heat treatment material.

In this embodiment, the bolt Wb shown in FIG. The manufacturing apparatus and the manufacturing process are the same as those in the first embodiment. The manufacturing apparatus shown in FIG. 1 was used, and processing was performed by the manufacturing process shown in FIG. That is, since the present embodiment is different only in the shape, component, diameter, etc. of the steel material to be processed, the heat treatment conditions are determined based on the same principle and the same parameters as in the first embodiment.

In the present embodiment, the case where the bolt Wb including the component shown in FIG. 40 is used as the processing target is illustrated, but the present invention is not limited to this.

FIG. 41 shows the heat treatment conditions for the bolt Wb in the heat treatment according to this embodiment and the comparative heat treatment. The heat treatment conditions of this embodiment (surface softening material) are as follows: frequency 50 kHz, quenching heating temperature 1000 ° C., tempering heating temperature 780 ° C., tempering heating time 0.15 s, and time from tempering heating to cooling 0.61 s. Yes.

The bolt Wb used here is a bolt having a diameter db = 7.1 mm, and the total cross-sectional tensile strength is adjusted to be about 1600 N / mm ² .

The comparative heat treatment conditions were a frequency of 9.5 kHz, a quenching heating temperature of 1000 ° C., a tempering heating temperature of 480 ° C., a tempering heating time of 0.61 s, and a time from tempering heating to cooling of 3.50 s. This comparative heat-treated material is also a bolt having a diameter of 7.1 mm, and the total cross-sectional average tensile strength is adjusted to be about 1600 N / mm ² . In addition, the component of the bolt Wb of the surface softening material of this embodiment and the component of the comparative heat treatment material are the same components. In addition, any heat treatment is performed before the screw is rolled into the bar material Wb1 of the bolt.

42 shows a simulation result of the relationship between the elapsed time of the heat treatment of the bolt Wb and the temperature change in this embodiment. Here, the relationship between the elapsed time and temperature according to the distance from the surface is shown.

42. As can be seen from FIG. 42, it is understood that the area of the hatched portion in the figure is larger in the surface layer portion than in the central portion. That is, in this surface layer part, it will be in the state hold | maintained for a long time at high temperature, and hardness will fall large.

FIG. 43 shows the hardness distributions of bolts (comparative heat treatment material) treated under comparative heat treatment conditions as comparative examples and bolts Wb (surface softening material) treated under the heat treatment conditions of this embodiment. . The vertical axis represents hardness [HV0.3], and the horizontal axis represents distance [mm] from the surface layer.

In FIG. 43, the hardness of the comparative heat-treated material is hardly changed even when the distance from the surface layer is changed. On the other hand, in the bolt Wb (surface layer softening material) subjected to the heat treatment according to the present embodiment, it can be seen that in the vicinity of the surface layer, the hardness changes as the distance from the surface layer increases. That is, Hv <500 in the range of 1.0 mm from the surface layer.

In the bolt Wb (surface softening material) treated under the heat treatment conditions of this embodiment, delayed fracture characteristics are improved as compared with the comparative heat treatment material.

FIG. 44 shows a table comparing delayed fracture test results of the bolt Wb (surface layer softening material) of this embodiment and the comparative heat treatment material. Here, the delayed fracture test was carried out after thread rolling the entire length of the bar material Wb1 of the bolt Wb. The depth of the screw was 0.7 mm. The conditions for the delayed fracture test were as follows: the test temperature was 50 ° C. in a state of being immersed in a 20% NH ₄ SCN solution, and a tensile strength of 1530 N / mm ² × 0.7 of the threaded portion was given. The test was conducted up to 200 hours using a loading method and a constant strain method. In the figure, “> 200” indicates that no fracture occurred even after 200 hours.

FIG. 45 is a graph showing the relationship between the cumulative rupture probability and the rupture time of the surface softening material and the comparative heat treatment material of the present embodiment. The abscissa indicates the cumulative rupture probability [%], and the ordinate indicates the rupture time [h].

44 and 45, it can be seen that the comparative heat-treated material fractured in 30 to 130 hours, whereas the surface layer softened material did not break even after 200 hours or more in all samples. Therefore, an improvement in delayed fracture resistance is recognized by performing the surface softening treatment.

Also in this embodiment, the same effect as the first embodiment can be obtained. That is, by combining surface heating by high-frequency induction heating and tempering performance of steel, a bolt excellent in delayed fracture resistance can be obtained by simple processing. That is, by using the temporal change of the temperature pattern at the moment of heating and the tempering characteristics, it is possible to obtain bolts having different hardness depending on the site by simple tempering that satisfies a predetermined processing condition. it can. Furthermore, the quenching and tempering heat treatment, which is rapidly heated by high frequency for a short time, provides high strength and high toughness as compared with heat treatment by ordinary furnace heating.

In addition, in this embodiment, although the component shown in FIG. 40 was shown as an example, it is not restricted to this.

Note that the present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying constituent elements without departing from the scope of the invention in the implementation stage. For example, specific processing conditions can be appropriately changed according to the shape, steel type, component, strength standard and hardness distribution to be obtained, and apparatus specifications. Also, the heat treatment conditions to be set are not limited to the above.

Here, as an example, in the first to fourth embodiments, a PC steel bar, a deformed PC steel bar, a spring steel wire, and a bolt are illustrated, but the same results are obtained by the processing using the same principle in any case. It has been. The concept of the present invention is not limited to these exemplified steel types, and can be widely applied to other types of steel materials.

Further, in each embodiment, the case where the steel material that has been strengthened by the quenching process is subjected to a tempering process to soften the surface layer has been described. A strengthened steel material may be used.

Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above embodiments. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

According to the present invention, it is possible to provide a steel material, a steel material manufacturing method, and a steel material manufacturing apparatus having different hardness depending on the site by a simple process.

Claims (15)

1. A method for producing a steel material in which the hardness of a part of the steel material is made lower than the hardness of the other part of the steel material by performing a heat treatment on the strengthened steel material,
   The heat treatment is a heating step of rapidly heating from the skin of the steel material to a certain depth by induction heating or direct current heating, and a cooling step of rapidly cooling the steel material after the heating step after a predetermined time after the heating step, With
   The method for producing a steel material, wherein a heating temperature in the heating step is equal to or higher than an Ac1 transformation point.
2. The method for producing a steel material according to claim 1, wherein the time from the heating step to the cooling step is a predetermined time or less determined according to the steel type, wire diameter, heating temperature, and heating time.
3. The method for manufacturing a steel material according to claim 1 or 2, wherein the treatment condition of the heat treatment is determined based on heat transfer characteristics after rapid heating of the surface of the steel material.
4. The processing condition of the heat treatment is a time integral value of the temperature of the steel material, and is determined based on a tempering progress value indicating a progress state of the tempering of the steel material. The manufacturing method of steel materials as described.
5. 5. The production of a steel material according to claim 1, wherein the treatment conditions of the heat treatment are a combination including at least two of frequency, input electric energy, heating temperature, heating time, and cooling time. Method.
6. A step of calculating heat transfer characteristics of the steel material or the tempering progress value;
   The method for manufacturing a steel material according to claim 4 or 5, wherein the processing condition of the heat treatment is determined based on the calculated heat transfer characteristic or the tempering progress value.
7. The treatment condition for the heat treatment is set such that the tempering progress value in the surface layer portion is 1.5 times or more of the tempering progress value in the center portion. Production method.
8. The time from the heating step to the cooling step is set so that the tempering progress value in the surface layer portion is 1.5 times or more the central tempering progress value. The manufacturing method of the steel materials in any one of 4 thru | or 7.
9. The method for manufacturing a steel material according to any one of claims 1 to 8, wherein the steel material is linear or rod-shaped.
10. The heating process and the cooling process are each performed once as a tempering process after the quenching process including the heating process and the cooling process is performed on the steel material. A method for producing the steel material according to claim 1.
11. The steel material having undergone the heating step and the cooling step according to any one of claims 1 to 10,
    Tensile strength when the difference between the hardness near the surface layer portion and the hardness on the center side from the position 10% from the surface layer in the radial direction is HV50 or more, and a tensile test is performed with a No. 2 test piece of JISZ2201 A steel material having a thickness of 1420 N / mm ² or more.
12. The steel material having undergone the heating step and the cooling step according to any one of claims 1 to 10,
    It has a full-section tempered martensite structure, the hardness of the surface layer portion is HV380 or less, and the tensile strength when a tensile test is performed with a No. 2 test piece of JISZ2201 is 1420 N / mm ² or more Yes,
    A steel material having a uniform hardness on the center side than the surface layer.
13. The steel material having undergone the heating step and the cooling step according to any one of claims 1 to 10,
    It has a full-section tempered martensite structure, the surface layer has a hardness of HV420 or less, and the tensile strength when a tensile test is performed with a No. 2 test piece of JISZ2201 is 1600 N / mm ² or more And
    A steel material having a uniform hardness on the center side than the surface layer.
14. An apparatus for producing a steel material, wherein the hardness of a part of the steel material is made lower than the hardness of the other part of the steel material by performing a heat treatment on the strengthened steel material,
    Heating means for rapidly heating the steel material from the skin to a certain depth by induction heating or direct current heating;
    Cooling means for rapidly cooling the heated steel material after a predetermined time after the heating,
    The apparatus for manufacturing a steel material, wherein the heating temperature of the steel material in the heating means is equal to or higher than the Ac1 transformation point.
15. The time from the end of the treatment in the heating means to the start of the treatment in the cooling means is a predetermined time or less determined according to the steel type, wire diameter, heating temperature, heating time, etc.
    The treatment conditions of the heat treatment are: heat transfer characteristics after rapid heating of the surface of the steel material, tempering progress value indicating the progress of tempering of the steel material, frequency, input electric energy, heating temperature, heating time, and cooling time. Determined based on a combination including at least two,
    The treatment conditions for the heat treatment are set so that the tempering progress value of the surface layer part is 1.5 times or more of the tempering progress value of the center part,
    The time from the heating step to the cooling step is set to a predetermined time or less determined according to the steel type, wire diameter, heating temperature, heating time, etc.
    The steel material is linear or rod-shaped,
    The apparatus further comprises conveying means for continuously conveying the steel material along a predetermined path passing through the heating means and the cooling means, and the processing condition includes a distance in the conveying path and the conveying speed,
    A quenching means for performing a heat treatment and a cooling treatment on the steel material;
    One each of the heating means and the cooling means are provided on the downstream side of the quenching means in the transport path,
    The steel material manufacturing apparatus according to claim 14, further comprising a control unit that controls processing conditions of the heat treatment based on a heat transfer characteristic of the steel material or a calculation result of the tempering progress value.

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